Arrangement for accelerating particles



March 6, 1962 N. F. VERSTER 3,024,379

ARRANGEMENT FOR ACCELERATING PARTICLES Filed Nov. 2'7, 1959 3 Sheets-Sheet 1 March'fi, 1962 N. FQVERSTER 3,024,379

ARRANGEMENT FOR ACCELERATING PARTICLES Filed Nov. 2'7, 1959 3 Sheets-$heet 2 I r d 20 v INVENTOR N. F. VERS TER L4 lP- AGENT March 6, 1962 N. F. VERSTER 3,024,379

ARRANGEMENT FOR ACCELERATING PARTICLES Filed Nov. 27, 1959 s Sheets-Sheet s INVENTOR N.l-'. VERsTER AGENT United States Patent M 3,024,379 ARRANGEMENT FOR ACQELERATKNG PARTICLES Nico Frederick Verster, Eindhoven, Netherlands, assignor to North American Philips Company, lino, New York, N.Y., a corporation of Delaware Filed Nov. 27, 1959, Ser. No. 855,651 Claims priority, application Netherlands Jan. 23, 1959 4 Claims. (til. 313--62) This invention relates to arrangements for accelerating particles, of the cyclotron type having a magnetic main field which is substantially independent of the azimuthal angle, comprising a device for extracting accelerated particles from the spiralised paths, having a regenerator ferromagnetic material which causes the magnetic field outside the spiralised paths to be increased through a small azimuthal angle.

In cyclotrons and synchro-cyclotrons of known type the accelerated particles are often extracted from the acceleration path by utilising only the aboveanentioned regenerator. It is also known to use, in addition, a socalled peeler which causes the magnetic field outside the spiralised paths to be decreased through a small azimuthal angle and which, as viewed in the direction of circulation of the accelerated particles, is situated before the regenerator, that is to say, as measured azimuthally, about 90 before the regenerator. Between this peeler and the regenerator there is located the mouth of the channel for the emergence of the particles.

In such arrangements the percentage of the particles which enters the channel of emergence and impinges on the target arranged behind it, is compartively low due to considerable radial spread of the particles occurring if the axial spread which likewise exists is maintained within reasonable limits. Consequently, in such known arrangements, in general, only about of the particles reach the target.

An object of the invention is to provide a cyclotron or synchro-cyclotron having a device for extracting the accelerated particles such that satisfactory radial concentration into a beam is obtained for a considerable proportion, for example at least 20%, of the particles, while retaining reasonable restriction of the deviations in the axial direction.

The arrangement according to the invention is characterized in that, as viewed in the direction of circulation of the accelerated particles, within an azimuthal angle of about 60 after the regenerator, there are provided means of ferromagnetic material for restricting the paths in an axial direction, these restricting means causing the decrease of the magnetic field upon increasing radius to be intensified through a small azimuthal angle in the region of the last path of the particles.

In order that the invention may be readily carried into effect, one embodiment will now be described in detail by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a plan view of part of an arrangement of the cyclotron type, of which only those elements are shown which are necessary for proper understanding of the invention.

FIG. 2 is a cross-sectional view along the radius 2-2 at right angles to the plane of drawing of FIG. 1, but on an enlarged scale.

FIG. 3 shows the variation in strength of the magnetic field along the radius 22 in the central plane of the cyclotron, along the horizontal axis on the same scale as FIG. 2.

FIG. 4 is a cross-sectional view along the radius 3-3 at right angles to the plane of drawing of FIG. 1, on the same scale as FIG. 2.

Patented Mar. 6, 1962 FIG. 5 shows the variation in strength of the magnetic field along the radius 3-3 in the central plane of the cyclotron, along the horizontal axis on the same scale as FIG. 2.

FIG. 6 shows, in rectangular co-ordinates, the cause of the radial paths of three particles during the end of the pre-ultimate revolution and during the ultimate revolotion, and

FIG. 7 shows, in rectangular co-ordinates, the cause of the axial paths of four particles during the end of the pre-ultimate revolution and during the ultimate revolution.

In FIG. 1, reference numeral 1 indicates the circumference of one of the pole-pieces of the cyclotron. One of the D-electrodes is indicated by 4. The second electrode 5 is in known manner of the dummy-D type. Between the two electrodes there is included an alternating voltage source which is shown diagrammatically at 6. The frequency of the alternating voltage applied is constant in a classic cyclotron, whereas it is varied in a synchrocyclotron.

The particles to be accelerated, such as protons, deutrons or alpha-particles are injected in known manner into the central portion of the cyclotron. By the action of the constant magnetic field, which is substantially at right angles to the plane of the drawing, together with the electric alternating field between the electrodes 4 and 5, the particles describe spiralised paths which, in the case under consideration, are traversed in the clockwise direction.

It is to be noted that the present invention relates to arrangements, the magnetic main field of which, that is to say, the field produced by the pole-pieces, is substantially independent of the azimuthal angle, so that the magnetic field is substantially of equal strength at all points of a circle about the centre 7, located in a plane parallel to the central plane.

For the extraction of the particles to be accelerated, there is firstly provided in known manner a regenerator 8 which also utilises correcting bodies 10, 11, 12, 13 positioned more inwards for the purpose of correcting the orbital travel of the particles. The regenerator and correcting means will be explained hereinafter with reference to FIGS. 2 and 3.

As viewed in the direction of reduction of the particles, about 30 further means 9 are provided for restricting the paths of the particles in an axial direction. These restricting means will be referred to hereinafter as the compressor. Also positioned more inwards are correcting bodies 14, 15 and 16. The compressor and the associated correcting bodies will be explained with reference to FIGS. 4 and 5.

The last equilibrium path described by a particle be fore being considerably influenced by variations in the field brought about by regenerator 8, compressor 9 and the associated correcting means is shown diagrammatically by a dotted circle 17. Subsequently, the particle describes a further number of paths (not shown) whereafter the particle, if sufiicient concentration is provided, is either extracted along the dotted path 18 or lost. Approximately at the area 18 it is possible to arrange a channel of emergence of magnetic material or a plurality of iron bodies in order to lead the discharged particles towards the target. Since such means are known and not essential to proper understanding of the invention, they are omitted in the drawing for the sake of simplicity.

Although it will be evident that the dimensions of the regenerator and of the compressor, as well as the relative positioning are dependent upon a number of parameters determined by the operational condition of the cyclotron, several possible dimensions and rules will be given hereinafter.

FIG. 1 shows one embodiment approximately to scale, in which the diameter of the pole-pieces is 300 cms. The largest radial dimension of regenerator 8 is about 12 cms. and the length of the regenerator, that is to say at right angles to the radius 22, is about 20 cms., so that the azimuthal angle of the regenerator in the present example is about 12.5.. The compressor 9 which, as measured azimuthally, is arranged about 30 further, has a largest radial dimension of about 3 cms., which is shown too large for the sake of clarity. In the azimuthal direction, the compressor has a length of about cms., which corresponds to an azimuthal angle of nearly 10. Insofar as reference is made to a variation in the magnetic field through a small azimuthal angle, this is to be understood to mean an angle which in practical cases is usually less than 30.

In FIG. 2, the reference numerals 19 and indicate parts of the two pole-pieces with their extreme edges 1. The regenerator 8 comprises two parts arranged symmetrically with respect to the centre plane 21 of the cyclotron. The correcting bodies 10, 11, 12 and 13 also comprise parts which are arranged symmetrically. As in the case of the regenerator 8, they are made of ferromagnetic material. It is common practice for the bodies 8, 10, 11, 12, and 13 to be rigidly secured to the polepieces 19 and 20 by means of non-magnetic member 34. The envelope 33 encloses the evacuated chamber.

The distance between the two pole-pieces 19 and 20, in a direction towards the outer edge 1, decreases in a stepwise manner in order to obtain a better course of the lines of force of the barrel-like type and hence an increase of the region serviceable for spiralised paths.

In FIG. 2, the equilibrium paths of the particles extend at the left-hand side of point 22, which corresponds to the circle 17 of FIG. 1. All the further paths of the particle extend in the region between points 22 and 23, the particle upon being extracted subsequently entering the zone outside point 24.

The curve shown in FIG. 3 shows the variation AB in the vertical component of the magnetic field in the centre plane 21 along the radius 22, which variation is brought about by regenerator 8 and the correcting means 10, 11, 12 and 13. The regenerator increases the magnetic field outside the spiralised paths. The decrease of the field which then occurs more inwards is corrected by the field of the bodies 10, 11, 12, 13.

In FIG. 4, the parts of the two pole-pieces are again indicated by 19 and 20. In the embodiment shown, the compressor 9 comprises one body of ferromagnetic material which is arranged symmetrically with respect to the centre plane 21. Each of the correcting bodies 14, 15 and 16 comprises two parts likewise made of ferromagnetic material. The compressor and correcting bodies are likewise secured to the pole pieces by member 34.

The equilibrium paths of the particles extend in the region to the left of point 26, the further paths of the particles passing through the region inside points 26 and 27.

FIG. 5 shows in full line the variation AB in the vertical component of the magnetic field in the centre plane 21 along the radius 3-3. The compressor 9 decreases the magnetic field outside the spiralised paths. The decrease of the field which occurs more inwards is corrected by the correcting bodies 14, 15 and 16. The curve of FIG. 5 shows a negative slope at the area of the last path of the particles, that is to say, in the region directly to the left of point 27 in FIG. 4. It is to be noted that there is no need to obtain an absolute decrease of the field at this area. If, for example, the correcting body 14 is larger, a variation is obtained as shown in dotted line in FIG. 5, in which event the field though being increased at the area of the last path, still exhibits a decrease upon increasing radius.

In FIG. 6, the horizontal axis indicates the circle 17 of FIG. 1 of the last equilibrium path having a radius r along which the regenerator, the compressor and again the regenerator are shown diagrammatically at 0, 30 and 360 respectively. The radial deviation r-r is plotted in a vertical direction. At the left-hand side of the regenerator at 0 there are shown the radial deviations of three particles a, b and c at the end of the last paths described by them before describing the path in which they are extracted. Between the paths of the particles a and c are the paths of about 40% of the particles of the beam. The particles in traversing the regenerator undergo a deviation which is strongly directed inwards. After part of one revolution, the paths of the particles before the regenerator reach a caustic focus, as shown at 28.

For the compressor which in this example comes an azimuthal angle of 30 after the regenerator, it is of primary importance to be positioned so that the particles still have a positive radial amplitude at the area of the compressor. With the paths of the particles as shown, it was still possible to position the compressor a little farther after the regenerator, viz. up to approximately an angle of 45. The maximum azimuthal angle which is still permissible in practice will be about 60", dependent upon the kind of the cyclotron.

FIG. 7 again shows, along the horizontal axis, the regenerator at 0 and 360 and the compressor at 30. The deviation in the Z-direction, the axial deviation, of the particles is plotted in a vertical direction. To the left of the regenerator at 0 there are shown the axial deviations of four particles, at the end of the last path before that in which they are extracted. The compressor causes the paths to be greatly bent backwards towards the centre plane so that the beam is also restricted sufficiently in the axial direction.

To ensure satisfactory performance of the arrangement it is desirable for the position of the compressor to be also chosen so that for the majority of the particles reaching the compressor with a positive slope, the axial amplitude is also positive and that for the majority of the particles reaching the compressor with a negative slope, the axial amplitude is also negative, and that these slopes have become smaller upon leaving the compressor. These requirements are fulfilled for the paths shown in the figures.

In relation to FIG. 6, it is still to be noted that with proper choice of the strength of the regenerator and of the compressor, the ratio between the various radial amplitudes of the paths at 28 is considerably closer to unity than at the end of the last path at 0. Due to the resulting smaller difference in amplitude and in relative phase shift, the particles either approach one another or describe paths which are more or less parallel. in either case the extraction of the particles towards a target is simpler. As previously mentioned, it is otherwise possible to use known expedients, such as a magnetic channel or means correcting the path, for leading the particles towards a target.

What is claimed is:

1. A particle accelerator comprising in combination an evacuated chamber, means to inject particles into said evacuated chamber, means for accelerating particles in said chamber in spiral paths including means for producing a magnetic field substantially independent of azimuthal angle and means to produce an electrostatic field, means to extract accelerated particles from said chamber, a regenerator of ferromagnetic material for producing a magnetic field in the region outside of the spiral paths which is increased through a small azimuthal angle, and ferromagnetic means for restricting the spiral paths in an axial direction positioned within an azimuthal angle of 60 after the regenerator whereby the decrease of the magnetic field with increasing radius is intensified through a small azimuthal angle in the region of the last path of the particles.

2. A particle accelerator as claimed in claim 1 in which the ferromagnetic means for restricting the spiral paths in 4. A particle accelerator as claimed in claim 3 in which an axial direction includes a magnetic field weakening the magnetic field producing means are a pair of pole member positioned to decrease the magnetic field outside members and the correcting bodies are attached to the the spiral paths. pole members by bodies of non-magnetic material.

3. A particle accelerator as claimed in claim 2 in which 5 References Cited in the file of this patent the ferromagnetic means Includes correcting bodles of ferromagnetic material located to correct the decrease in UNITED STATES PATENTS the magnetic field in an inward radial direction. 2,812,463 Teng et a1 Nov. 5, 1957 

